The 802.11 standard defines in the document “IEEE 802.11a-1999, IEEE 802.11b-1999, IEEE-802.11d-2001, Part 11: wireless LAN medium access control (MAC) and physical layer (PHY) specifications” a method for regulating traffic in a wireless network. This method uses a system of congestion windows (Congestion Window “CW”) to regulate this traffic. According to this standard, to determine the instant at which to send a data packet, a station draws a chance random number between 0 and CW−1, the value CW being an integer lying between two values CWmin and CWmax specified by the 802.11 standard.
This value CW serves as countdown counter for the sending of the packet, this counter being deferred if the station notes that another station is currently sending. Unfortunately, this congestion windows system generates a significant number of collisions on the wireless network, this being manifested, from the user's point of view, by a significant loss of bandwidth.
Another mechanism known by the name “tournament scheme” may be used by the various stations to regulate the sending of packets and limit the collision rate. This tournament scheme is described in the document by the authors Z. Abichar and M. Chang, entitled “CONTI: Constant Time Contention Resolution for WLAN Access”, IFIP Networking 2005.
The tournament scheme consists in organizing a sort of tournament between the stations having to send a packet. A tournament is composed of a certain number of selection rounds, each executed for a time interval of predefined duration allowing each station to have time to hear the sending of any other. This duration can typically be equal to the duration of the time interval dubbed “SlotTime” which, according to the IEEE 802.11 standard, is defined as an elementary interval in the congestion window contention resolution procedure. At the start of the tournament, all the stations having to send a packet may be permitted to send this packet. At each selection round, one or more stations may be deleted from the list of the stations permitted to send, depending on whether a permission or a prohibition to send is allotted to them during this selection round. On completion of the tournament, only the stations not eliminated are permitted to send. If several stations remain in the running on completion of the tournament, they send at the same time thus causing a collision and therefore disturbed reception with impossibility of correctly receiving the data packets sent. These stations will then have to participate in the next tournament to attempt to send these packets again.
The tournament scheme in a WLAN network allows effective use of bandwidth and a noticeable reduction in collisions and access time, thereby rendering these networks compatible with multimedia applications requiring high transmission bitrates.
However the effectiveness of this scheme is reduced in the presence of the phenomenon of hidden stations. This phenomenon arises for example when 3 stations A, B, and C are located in such a way that station B hears the signals sent by stations A and C, but that the latter do not hear the signals sent by the other. In such a situation, if station A is permitted to send a packet and sends a signal indicating its intention to send, station C will not hear this signal. Consequently, if station C is also permitted to send a packet, it will send this packet, believing it to be the only one to send a packet, and will cause a collision with the packet sent by station A.